I think the Unlimited ammo stopped working after this last Patch which added more DLC.
Even though in mission I'm showing 1000 rds my heavies are running out now unlike before
this game is patching 2 times a day (after this DLC update). I guess the developers broke the title with their DLC and are having to fix it. we have this on the list of things to do this week. we are going to wait a bit to see if they can fix their broken DLC..
best,
cal